SINGAPORE - Multiple studies suggest that having adequate amounts of vitamin D may play a role in helping people stave off or combat the coronavirus, although the jury is still out on whether the results are conclusive or why this is so.
Commonly known as the "sunshine vitamin", vitamin D, which is known for its immune-boosting function, is mainly made in the skin when it is exposed to the ultraviolet B rays of the sun. It can also be obtained from other sources such as eggs, liver and oily fish.
At least one overseas study has associated vitamin D deficiency with a higher risk of Covid-19.
Published in medical journal JAMA Network Open on Sept 3, the study observed 489 patients from the University of Chicago Medicine health system, about a third of whom had vitamin D deficiency.
Patients with vitamin D deficiency and who were not given treatment for it were 1.77 times more likely to test positive for Covid-19 than those who were not.
The study also noted that other research had found that Covid-19 was less prevalent in groups that had lower rates of vitamin D deficiency.
Lockdowns and other measures to reduce the spread of Covid-19 could also decrease exposure to the sun, the researchers pointed out.
They added: "The low costs of vitamin D and its general safety... support arguments for population-level supplementation, perhaps for targeting groups at high risk for vitamin D deficiency and/or Covid-19."
A similar call was made in medical journal The Lancet on Aug 3 by researchers from the University of Cambridge and the Queen Mary University of London.
Citing several other studies on the subject, as well as vitamin D's ability to protect against other acute respiratory infections, the researchers called for more trials to investigate whether the vitamin could help reduce the severity of Covid-19.
They also suggested increasing efforts to ensure members of the public have sufficient vitamin D.
"There is a chance that (such efforts) might also reduce the impact of Covid-19 in populations where vitamin D deficiency is prevalent; there is nothing to lose from their implementation, and potentially much to gain," they wrote.
Closer to home, another study analysed 43 Covid-19 patients aged 50 and above at Singapore General Hospital.
Published in the science journal Nutrition last week, it found that treating such patients with a combination of vitamin D, magnesium and vitamin B12 was associated with a "significant reduction" in the number of those who went on to require oxygen support or admission to intensive care.
While experts here acknowledged the legitimacy of such studies, they called for caution and said that more research is required.
Dr Ben Ng, an endocrinologist from Arden Endocrinology Specialist Clinic at Mount Elizabeth Novena Specialist Centre, said the current data may appear to support the role of vitamin D in tackling the coronavirus, but that "robust clinical data" is needed, given how new the disease is.
He noted also that the pandemic will likely have resulted in people here having decreased levels of vitamin D, given the reduction in outdoor activities.
"Considering that it will benefit musculoskeletal health and may help with overall Covid-19 risk, I would encourage all patients who may be at risk of vitamin D deficiency to consider taking vitamin D replacement," he said, adding that a daily dose of 400-1,000 units is "extremely safe and unlikely to cause harm".
Infectious diseases expert Leong Hoe Nam, also from Mount Elizabeth Novena, agreed.
"We don't fully understand the reasons why vitamin D might work against Covid-19 - but given that it is harmless if taken within the recommended dosage, very cheap, and can be obtained free of charge via the sun or taken easily through oral supplements, it makes sense to promote its intake in those who have vitamin D deficiency," he said.
Dr Gail Cross, consultant at the National University Hospital's Division of Infectious Diseases, took a more conservative stance.
"In the scientific community, statistical significance seen in these studies is taken with a grain of salt. If anything, the most these types of studies provide us with is evidence to look at conducting a prospective study to examine the question of how vitamin D impacts Covid-19... We all need to wait for more and better quality evidence," she said.
Too much vitamin D can also cause toxicity and can lead to an excess of calcium in the body, which can be dangerous, Dr Cross warned.
"I would discourage anyone from taking supplemental vitamin D to prevent or treat Covid-19 unless specifically directed by their doctor in the instance of a deficiency," she said.
With fat-soluble vitamins like vitamin D, more is not necessarily better, said an expert in dietetics and nutrition at the Singapore Institute of Technology, Assistant Professor Verena Tan.
"They can build up in the body and may cause side effects and toxic build-up. Always check with a doctor or dietitian before starting a vitamin D supplement," she said, adding that taking vitamin D supplements alone cannot protect one from Covid-19.
However, she noted that existing evidence shows having healthy vitamin D levels can enhance immunity, and that the vitamin is required by the body for calcium absorption and maintaining bone structure.
"There is also emerging evidence showing that vitamin D is critical for immune function and a deficiency may compromise immune response and increase risk of infection and disease," she said.
Dr Cross agreed that treating vitamin D deficiency can be beneficial to one's overall health, which will have the spillover effect in helping with preventing or treating Covid-19.
She said: "So perhaps for members of the public, the first step might be to determine if you have vitamin D deficiency and whether your overall health would benefit from supplementation."